Memory alteration magic sources

I've knotted together a scenario, where I would like a bit of help unraveling.

TL;DR backstory of one of my players is that he was orphaned/abandoned as an infant and now he's looking for his parents (or mother actually, since the party killed his father... or the undead form of his father). Aaaaanyway... Recently, this character got possessed by my homebrew monster who took him to Shadowfell, where he eventually killed the monster. In order to return to the Material Plane, he had to solve a puzzle and in order to solve the puzzle he had to pay a price... a memory. The memory the player chose was that when he was a kid he went to the market with his mother and he wanted some new clothes, which his mother didn't get him (he's also quite a fashion "guru", always crafting clothes for himself). This lead into the situation that the prize to leave Shadowfell was the memories of his mother. He remembers him being at the market with someone, but he has no idea with whom and so on. This slightly contradicts his original backstory of being abandoned as a baby.

They'll soon have an opportunity to meet with a devil, who will very kindly offer to return this character's memories to him. Only for the devil to also realize that it's not the first time the character's mind has been tampered with. A devil won't admit they can't do something, so I need to plan out his roleplay accordingly.

Because of the mismatch between the original backstory and the memory the player chose, I decided that he does indeed remember something of his childhood, but they are affected by some other magic. I don't want to use another Shadowfell creature nor another devil. I thought the mother went to a hag to remove her sense of remorse, so that she can leave the baby and advance her own career. At the same time the hag altered the character's mind, so that he would simply forget to think about his parents. In place, the hag would place alternative memories into his mind to fill out gaps.

Now my question is this: which creatures have this sort of memory alteration magic available for them already? Which type of creature would most likely agree into something like this? Even if it is a ready monster, I will probably homebrew it slightly to fit into the story. I'm preferably looking something like a CR 5-12 monster, so that I don't have to homebrew too heavily. Any suggestions?

Not sure could make it a feat or spell. 

Made this for you Memory alteration

Thanks, but that's not really what I asked.

Sorry

Mind flayers, Aboleths, and Oblexes could all work. Or Amnizus, but they have a higher CR than what you said.

An Amnizu is "just another devil", which could work. Initially I wanted to avoid using another devil to involve different types of creatures. I got the idea for the hag from VLDL D&D campaign, but there wasn't really a good hag base with the right CR range.

A Mind Flayer could work thematically, but another campaign where I'm the player as is one of my players (my brother) has a big theme around Mind Flayers + they're a major thing in Baldur's Gate 3 (which again is a game me and my brother are actively playing). I don't necessarily want to use them for this side adventure as the campaign doesn't have otherwise anything to do with them.

I know nothing of Aboleths or Oblexes, but just with a quick glance at their stat block I can't see a reason why either of them would do agree to do this. I think the major question is why would a creature agree to do it. The mother wants to not feel regret of what she's about to do, so she's looking for _a creature_ to do it for her. Outside a devil and hag, I can't really think of creatures that would agree to do something like this. And who would also "be available", meaning that the creature is not only encountered in another plane of existence or in a vastly different habitat on the Material Plane than she already is (which is between Sword Coast and Anauroch).

Why not just homebrew someone? Messing with memories and taking kids from their families is classic fey move. Could easily be some kind of archfey. 
Best thing is archfey can be super-fickle and flighty if you want them to be. Its motivation doesn’t have to be beyond, it found the idea amusing at that moment. It might not even remember having done it, as it was such a trivial thing at the time.

Any psychic creature would be a good option- a gem dragon might work.  They're not usually evil, but they're also not necessarily nice.  An Adult Crystal Dragon is CR 12.

An NPC spellcaster could potentially do this as well.

5e stat blocks are mostly about combat capabilities so they often don't mention weird things. A hydroloth is another thing that messes with memories (though mostly just stealing them). However, anything with the modify memory spell can do this (possibly requiring a higher level spell slot), which gives you a pretty broad range of creatures that could do something of the sort.